This application pertains generally to signal processing circuitry in a radar receiver and particularly to an adaptive narrow band filter capable of locking on and tracking a signal having a frequency range wider than the bandwidth of such filter.
It is known in the art that a continuous wave radar may be made capable of ranging on targets by appropriate modification of the waveform of the signal transmitted by such a radar. One known way of accomplishing such an end is periodically to frequency modulate a continuous wave carrier signal with a sinusoidal waveform so that a comparison between the instantaneous frequency of the sinusoidal waveform and the frequency of echo signals may be made to indicate the approximate range of each detected target. A radar with such coding of the transmitted signal will be referred to hereinafter as an FM-CW radar.
In the semiactive missile guidance system shown in the copending patent application entitled “Adaptive Semiactive Missile Guidance System and Elements Therefor”, Ser. No. 579,291, inventors Donald S. Banks, George R. Spencer and James Williamson, filed on May 20, 1975 and assigned to the same assignee as this invention, an FM-CW radar is used as a control radar to illuminate both a target and a guided missile in flight to allow the latter to process radar signals to derive guidance signals. In the processing of echo signals on the guided missile final demodulation is accomplished by applying the downconverted echo signal to a phase detector along with a reference signal from a voltage-controlled oscillator. The frequency of the voltage-controlled oscillator is controlled through a feedback circuit incorporating a filter (which has a relatively wide passband under some conditions and relatively narrow passband under other conditions) to null the output of the phase detector. When the filter has a relatively wide passband, i.e. wider than any frequency modulation present on the signal being demodulated, the frequency modulation on the down-converted echo signal is passed to the voltage-controlled oscillator with practically no change in amplitude or phase. The reference signal out of the voltage-controlled oscillator then is caused similarly to vary, ultimately then to null the output of the phase detector. A different situation obtains, however, when the filter has a relatively narrow passband, i.e. narrower than the frequency modulation on the signal to be demodulated. In that case the frequency modulation on the down-converted echo signal is blocked, more or less, or shifted in phase by the filter. Then, because the reference signal out of the voltage-controlled oscillator is not modulated with a replica frequency modulation on the downconverted echo signal, the output of the phase detector cannot be nulled. That is to say, the frequency modulation on the downconverted echo signal appears at the output of the phase detector as an error signal at the repetition frequency of the FM-CW radar.
An attempt was made in the referenced system to solve the problem being discussed. The approach taken was to detect the frequency modulation on the downconverted echo signal and then, after filtering, to apply the resultant (along with the signal out of the filter in the feedback circuit) to the voltage-controlled oscillator. While, in theory, the just-outlined approach should be satisfactory, it has not so proved to be. The difficulty experienced in practice derives from the fact that proper filtering of the detected frequency modulation is almost impossible to achieve under all conditions. That is, when the signal-to-noise ratio between the downconverted echo signal and noise accompanying such signal is low, there is no practical way of reducing the noisiness of the detected frequency modulation to an acceptable degree.